NEUROMAGNETIC MEASUREMENTS OF THE HUMAN PRIMARY AUDITORY RESPONSE

The spatio-temporal organization of the human primary auditory respons e (N19 and P30) was determined by analyzing magnetic fields from the r ight hemisphere evoked by monaural clicks to the left ear. The magneti c response consisted of peaks corresponding to electrical N19 (N19m) a nd P30 (P30m) within 40 msec after stimulation. However, the onset of electrical N19 occurred 5-9 msec earlier than that of the magnetic cou nterpart. Furthermore, the relative amplitude of N19 and P30 (N19/P30) was larger (more than 1) than that (about 0.5) of N19m and P30m (N19m /P30m). The findings suggest an additional contribution from a subcort ical source for electrical N19. The estimated equivalent sources for N 19m and P30m were localized at the postero-medial part of Heschl's gyr us (the primary auditory cortex). Furthermore, the dipole orientation of N19m was directed postero-ventrally and that of P30m was in the opp osite direction, which is compatibile with the anatomy of the supratem poral primary auditory cortex. These results resolve a long standing c ontroversy of cortical vs. subcortical structures as the generators of the N19.